Means and method for controlling output from crystal microphones



Dec. 30, 1941. F. M. SCHMIDT A i (2,267,376

MEANS AND METHOD FOR CONTROLLING OUTPUT FROM CRYSTAL MICROPHONES Filed July 15, 1939 INVENTOR ATTORNEY Patented Dec. 30, 1 941 1 i UNITED STATES PATENT- OFFICE MEANS AND METHOD FOR. CONTROLLING I OUTPUT FROM CRYSTAL MICROPHONES Francis M. Schmidt, North Tonawanda, N. r.,' assignor to The -Rudolph Wurlitzer Company, North Tonawanda, N. Y., a corporation of Ohio Application July 15, 1939, Serial No. 284,689

3 Claims. (Cl. 179-1) This invention relates to certain new and use- Referring now to the drawing, wherein my ful improvements in the means and method for invention has been illustrated, by way of excontrolling the output from a crystal microphone. ample, in association with an electro-acoustical It has for its chief object to provide a novel system, the numeral l indicates the crystal system for controlling the output from a crystal microphone mounted on a suitable standard ll, microphone at the microphone, irrespective of the and 12 indicates generally the reproducing apparemoteness of the associated electro-acoustical ratus or amplifier which is located remotely from system from the microphone. the microphone and which is provided with a Another object of the invention is to provide a suitable loud speaker unit l3 of well known conmicrophone control of this character which is so struction. Connecting the reproducing apparatus designed as to effectually and faithfully control with the microphone is a cable l4 which may be the volume of the microphone without the 00- of any length and which contains the wires of currence of frequency discrimination, and which the control circuit. Associated with and applied control provides for a uniform volume level to the microphone is a switch l5 and a volume whether the electro-acoustical system be located control knob I6, where y he user can readily near or at a. remote distance from the microcontrol, directly at ,the microphone, the switchphone. ing of the microphone "on and off as well as Other features of the invention reside in the govern its volume output. construction and arrangement of parts herein- The cry cr ph ne I0 is connected by the after described and particularly pointed out in wire I! with the con grid Of Variable, the appended claims. mutual conductance amplifier or electron tube In the accompanying drawing: I9 constituting a part of theamplifler system l2.

Figure 1 is a side elevation of the improved The amplifi t n of t s u is varied y microphone control associated diagrammatically changing its bias volta e by m ns of the volwith an amplifier and speaker system. Figure 2 ume control knob 16 which is operatively conis a. diagrammatic view of the circuit embodying nected to a, variable resistor 20 of any well known my invention. type, that shown in the drawing including the Similar characters of reference indicate correresistors 2 and adjustable control element 22 spending parts throughout the several views. to which the knob l 6 is connected. Leading from Heretofore, when it has been desired to operthe adjustable control element 22 of the resistor ate a crystal microphone and to have the volume is Wire 23 Which is connected through fi ter of the crystal microphone and its associated elements 24, 25 and 25 t0 the ode 21 of t e electro-acousticai system controllable from electron tube. The function of these filter elepoint near to or within rea h of th microphone ments is to eliminate electrical noise as the volwhen. h associated governing equipment ha ume control is varied and'to stabilize the amphbeen l ated t a point remote fro th microfier. circuit to prevent degeneration therein. I phone, it ha b n necessary t u a i bl The variable resistor 2| is so connected that condenser as a volume control unit because of a constant fixed Voltage s aintained at all the capacitative nature of a crystal microphone times, and to this nd one terminal thereof is unit. Such variable condenser controls, however, 40 connected y a 28 through the cable It to are bulky and expensive, and it is therefore d a resistor 29 and thence to the supply voltage or sirable to create a new method for controlling t y 39 0f the p r ni ii. The return the volume of the crystal microphone directly circuit from this supply voltage is carried through t t micrgphone the cable M by a wire 3| connected to the other In this connection, if aresistive control were terminal of the Variable resistor element 2|. to be used the frequency characteristic would When e djustable control element 22 of th change with the volume control and the response resistor is ehanged y means of the control knob would not be suitable nor faithful. If an elect e bias of the tube I9 is a ered hus eha tricalnetwork were used to correct for this freing the mplifica ion of the tube and the frequency discrimination, it would be. expensive and quency characteristic of the amplification circuit suitable only as long as the connecting cable befollows that of the crystal microphone for all voltween the crystal microphone and the remotely ume levels. The cable l4 between the microassociated acoustical system was not lengthened phone and the associated amplifier system may nor shortened, in which case frequency discrlmy in l h a no frequency discrimination ination would exist. occurs.

The amplifier system I2 which is located remotely from the microphone and connected thereto is of a conventional type having audio amplifier or electron tubes 32, 33,14 and 35 connected to the speaker I3.

I claim as my invention: v

1. In combination, .a crystal microphone, an electro-acoustical systemassociated therewith for operative connection thereto. an amplifying circuit for the microphone operatively connected to said electro-acoustical system and including a variable, mutual conductance electron tube having its control grid connected to said microphone,

a variable resistance associated with said ampli fying circuit and connected to the cathode of said tube for varying the amplification thereof, and means located adjacent the microphone for controlling said variable resistance.

2. The combination with a crystal microphone and an associated electro-acoustical system located remotely therefrom, of a circuit operatively connected to said electro-acoustical system and including a variable, mutual conductance electron tube having its grid connected to said microphone and its output circuit coupled to said acoustical system, and a variable resistance operatively connected to the cathode of said tube for varying its amplification and having a control element therefor positioned adjacent to said microphone.

3. The combination with a crystal microphone and an associated electro-acoustical system located remotely therefrom, of a circuit including a variable, mutual conductance electron tube constituting a part of the electro-acoustical system and having its'control grid connected to the microphone, and a variable resistance connected to'the cathode of said tube and having a control element therefor for varying theampliflcation of such tube by changing its bias voltage.

FRANCIS M. SCHMIDT. 

